Water-absorbing resin (Super Absorbent Polymer or SAP) is a water-swellable, water-insoluble polymer gelling agent. Water-absorbing resin is used for various applications. For example, water-absorbing resin is used in absorbent articles such as disposable diapers, sanitary napkins, and incontinence articles for adults, agricultural and horticultural water retaining agents, industrial waterproofing agents, and the like. Many monomers and hydrophilic polymers have been proposed as raw materials of such water-absorbing resin Front the viewpoint of performance and cost, a polyacrylic acid (salt)-based water-absorbing resin containing an acrylic acid and/or a salt thereof as a monomer(s) is most often used as such water-absorbing resin
The above water-absorbing resin is produced through steps such as a polymerization step, a drying step, a classification step, a surface-crosslinking step, and an addition step of adding one or more of various modifiers (Non-Patent Literature 1). Further, along with enhancement in performance and reduction in thickness of disposable diapers which are one of major applications of water-absorbing resin, water-absorbing resin is being required to have higher performance in terms of, for example, fluid retention capacity (e.g., CRC and FSC), gel strength, water-soluble component, water absorption speed (e.g., FSR and Vortex), fluid retention capacity under pressure (e.g., AAP), liquid permeability (e.g., SFC and GBP), particle size distribution, urine resistance, antibacterial property, impact resistance, powder fluidity, deodorizing property (e.g., pHAI (=(CRC+AAP)×(7−pH)), coloration resistance, and low dustiness. In addition, in general, improvement in performance of water-absorbing resin is incompatible with improvement in productivity of water-absorbing resin. It has been always an important issue to concurrently improve both performance and productivity of water-absorbing resin.
In view of the above issue, with regard to each step from the polymerization step to the addition step of adding one or more of various modifiers, many improved techniques have been proposed as measures to improve performance of water-absorbing resin. In particular, many proposals have been made with regard to improvement of the surface-crosslinking step and development of modifiers (Patent Literatures 17 through 37). In addition to the above conventional improved techniques, in recent years, technical improvement has been made for the classification step. Specifically, proposals have been made on measures to improve productivity and performance of water-absorbing resin by prevention of aggregation during classification and improvement of classification efficiency (Patent Literatures 1 through 15).
The following techniques are the proposals which have been proposed so far as measures to improve performance and/or productivity of water-absorbing resin in the classification step: a technique in which electricity is removed from a classification apparatus (Patent Literature 1); a technique in which a stretch tension of a classification mesh is set within a predetermined range (Patent Literature 2); a technique in which tapping balls are provided below a classification mesh (Patent Literature 3); a technique using a plurality of classification meshes that are substantially identical in mesh size (Patent Literature 4); a technique in which classification is carried out under airflow (Patent Literature 5); a technique in which classification is carried out under reduced pressure (Patent Literatures 6 and 7); a technique in which a classification apparatus is heated and heat in the classification apparatus is retained (Patent Literature 8); a technique in which a guide is provided on a surface of a classification mesh (Patent Literature 9); a technique in which classification is performed both before and after surface-crosslinking (Patent Literature 10); a technique using a plurality of sieves (Patent Literature 11); a technique in which a classification apparatus is vibrated at a predetermined number of vibrations (Patent Literature 12); a technique specifying a second classification step after addition of water and a conveying step subsequent to the second classification step (Patent Literature 13); a technique specifying a circulation ratio of water-absorbing resin in pulverization and classification steps (Patent Literatures 14 and 15); and the like.
Moreover, there has also been a disclosure of a water-absorbing resin classification method in which a classification operation is additionally performed two times after surface-crosslinking and in the second classification operation after the surface-crosslinking, a Ton-Cap sieve having a specific mesh size is used (a classification mesh used in the second classification operation after the surface-crosslinking has a larger mesh size in a direction of long sides of holes of the classification mesh, as compared to another classification mesh used in the first classification step after the surface-crosslinking) (Patent Literature 16).